It is known to provide electric shielding for wires to reduce the effects of electromagnetic radiation, both from the standpoint of coupling radiation into a electrical wire or device, and from the standpoint of preventing radiation emissions from an electrical wire or device. An electric field shield is typically obtained by placing an electrically conductive layer of material in electrical isolation around the electrical wire or device and connecting the conductive layer to ground. The conductive material may be, for example, a film, sheet, wire braid or wire mesh made of copper, aluminum or the like.
Wire braid is commonly used to shield electric cables and wires. One problem with the copper or tinned copper braid type of shield is that it does not attenuate magnetic fields. Rather, it reflects an incident magnetic field and may pass up to 90% of the incident magnetic field. This magnetic field can in turn induce currents which interfere with the normal operation of devices that are subjected to the passed magnetic field or connected to wires that are subjected to the passed magnetic fields.
Most wiring currently used in an automotive vehicle is copper wire. Due to the nature of the automotive vehicle, and its mechanical and electrical devices, there are large transient and cyclic current discharges. These discharges produce correspondingly large electromagnetic fields during the normal starting and running operation of the vehicle. These electromagnetic fields will interfere with the operation of the vehicle electronics and the electronic systems of adjacent vehicles or devices if the electronics and wiring harnesses are not properly shielded.
It is known to wrap electrical wires and devices with a magnetic material to shield the wire or device from magnetic fields. One problem with this technique is that the magnetic fields leak out the ends of the wrap and may leak through seams in the wrap. It is commercially impractable to wrap completely a magnetic shielding material about electrical devices and wires and to weld the seams closed.
Another known approach to shielding electrical devices uses laminate boxes to surround the electrical device. The laminate includes an outer layer of copper, a middle layer of stainless steel (e.g., type 430), and a inner layer of copper. The copper layers are secured to the stainless steel by interatomic bonding, e.g., electroless plating. The stainless steel has a permeability that acts as a magnetic shield. Such a laminate structure is available from Texas Instruments under the trade name TI-SHIELD.
One problem with the laminate sheet structure is that it is not suitable for shielding wires. In particular, the rigid laminate structure is not easy to wrap around wires of particularly small diameter or to shield structures that are not boxlike. Another problem is that the laminate structure prevents the stainless steel magnetic shielding material from forming a good stainless to stainless contact and a tight magnetic field seal. The copper layer to copper layer contact provides a magnetic field leakage path ground the edge of the stainless steel layer. Wrapping such a laminate helically around a cable also forces the magnetic flux to follow a helical path around the cable.
In the automotive environment, magnetically shielding wiring harnesses by the known techniques is particularly difficult because the shield must be installed on the wiring before the wiring is manipulated into place on the vehicle.
It is therefore an object of the invention to provide an improved magnetic field shield for wires and other devices that does not suffer from the defects of the known magnetic shields and shielding methods.
It is another object of the invention to provide a magnetic and electric field shield for flexible electric wires and cables. It is yet another object of the invention to provide a magnetic and electric field shield for electric wires suitable for use in an automotive vehicle.